This invention relates to an apparatus for forming underground pipelines of the type which include, at least during construction of the pipeline, an inner component at least partially enclosed by a formable outer component. The inner component typically includes a plurality of inner component sections joined in end-to-end relationship. Each of the inner component sections may be, for example, a pipe section, a core unit, or an inner form. If a pipe section or core section is used, such section remains as a permanent part of the pipeline. However, if a form is used, it is removed after the outer component is applied, and in this event, the form is part of the pipeline only during construction of the pipeline.
The formable outer component may be of a variety of flowable materials used to encase, or partially encase, the inner component. For example, the formable outer component may be a cementitious material, such as concrete or other materials, which is flowable and curable to form a casing, or partial casing, for the inner component. Alternatively, the formable outer component may be a cushion material or a noncurable material such as sand or soil. The resulting pipeline can be made capable of carrying various materials, including liquids.
My prior U.S. Pat. No. 3,830,606 discloses an apparatus for forming underground pipelines of the type described above. The patented construction includes a recetptacle for receiving an inner component section and a bulkhead at one end of the receptacle. The bulkhead has an outlet opening so that the inner component section can be passed through the outlet opening and be joined to a previously laid inner component section. A hopper for storing a supply of the formable outer component is mounted on the bulkhead. Thus, the formable outer component in the hopper forms a source of the outer component. In the preferred embodiment of U.S. Pat. No. 3,830,606, the formable outer component is flowed downwardly from the hopper to a region rearwardly of the bulkhead so it can enclose the inner component. Another function of the hopper is to provide a hold-down mechanism for the inner component so that it will not be elevated above a predetermined position in response to various forces acting on the inner component section, such as the flotation force exerted on it by the formable outer component.
The patented method and apparatus are very satisfactory. However, the emptying and filling of the hopper cause the apparatus to have a substantially variable mass which, in certain applications, could move or gouge the inner component section or create adverse grade control as the apparatus advances. In addition, the formable outer component is dispensed by the hopper from above the inner component section, and this can cause air to be trapped beneath the inner component with the result that the casing around the inner component contains voids.